This invention relates generally to nuclear reactors and more particularly, to apparatus and methods for repairing piping within reactor pressure vessels of such reactors.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure.
Boiling water reactors have numerous piping systems, and such piping systems are utilized, for example, to transport water throughout the RPV. For example, core spray piping is used to deliver water from outside the RPV to core spray spargers inside the RPV. The core spray piping and spargers deliver water flow to the reactor core.
Intergranular stress corrosion cracking (IGSCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high temperature water. The reactor components are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment and radiation can increase the susceptibility of metal in a component to IGSCC.
Conditions exist in the reactor which contribute to IGSCC of the core spray piping. One area of susceptibility in the core spray piping is the welded joints between the upper T-box and its associated distribution headers. The upper T-box is the junction where the core spray supply header branches to distribution header pipes. The core spray system prevents excessive fuel clad temperature in the event of a Loss of Coolant Accident (LOCA) by delivering cooling water to the core region of the reactor. In the event that through-wall circumferential cracking should occur at these welded joints, the system may be compromised.
In order to prevent unacceptable leakage and to ensure that the core spray system delivers the necessary volumetric flow rate to the reactor core, it would be desirable to provide a clamping system to provide structural integrity to the T-box and to hold the welded joints together in the event that one or more welds fail.
These and other objects may be attained by a T-box clamp apparatus which applies compressive forces to the T-box welded joints to hold the distribution headers in place and in contact with the T-box. The T-box clamp apparatus includes a first and a second clamp assembly and a clamp strong-back assembly coupled to the first and second clamp assemblies.
The first and second clamp assemblies each include an upper clamp body, a lower clamp body, and a clamp bolt and nut assembly. The upper and lower clamp bodies are configured to conform to the curvature of the header pipes. Each upper and lower clamp body also includes a clamp bolt opening extending through the clamp body and configured to receive the clamp bolt. Particularly, each upper and lower clamp body has a flat side and a curved side configured to conform to the header pipe. The flat side includes a flange depending from the clamp body. The flange is located so as to divide the flat side into two flat portions. One of the flat portions includes a groove extending longitudinally from the end of the clamp body to the flange.
The strong-back assembly includes an upper strong-back body, a lower strong-back body, and a cover plate. The upper strong-back body is configured to conform to the T-box and to couple to the upper clamp body of the first and the second clamp assemblies. The lower strong-back body is configured to conform to the T-box and to couple to the lower clamp body of the first and of the second clamp assemblies. Particularly, each strong-back body includes an elongate member having a curved middle portion and two flat end portions. The inner side of the curved middle portion is configured to conform to the T-box. Each edge portion includes a tongue extending longitudinally from the outer side of the edge portions. Each tongue of the upper strong-back body is configured to engage the groove of an upper clamp body to form a tongue and groove joint. Each tongue of the lower strong-back body is configured to engage the groove of a lower clamp body to form a tongue and groove joint. Each upper clamp body is coupled to the upper strong-back body by a cross-bolt, and each lower clamp body is coupled to the lower strong-back body with a cross-bolt. Particularly, the flange of each clamp body includes a bolt opening extending through the flange. The bolt opening is configured to receive a cross-bolt and to be in substantial alignment with a bolt opening located at each end of the upper and lower strongback bodies. Each strong-back body bolt opening is configured to threadingly engage a cross-bolt.
The T-box clamp apparatus also includes a cover plate configured to engage the welded cover of the T-box. The cover plate is flat having a substantially cross shape and is configured to couple to the upper and the lower strong-back bodies. Particularly, cover bolts extend through bolt openings in the cover plate. The bolt openings are configured to align with threaded bolt openings in the upper and the lower strong-back body. The cross shape of the cover plate permits for visual inspection of over one-half of the T-box cover weld.
To assemble the T-box clamp apparatus on the upper T-box in a boiling water nuclear reactor, a hole is first machined in each distribution header pipe extending from the upper T-box. The holes are located adjacent the T-box and are configured so as to align with the clamp bolt openings in the clamp bodies of the clamp assemblies and to receive a clamp bolt. The upper and lower clamp bodies of each clamp assembly are then positioned on the distribution header pipes and each clamp assembly is clamped to a distribution header pipe by a clamp bolt and nut assembly.
The upper strong-back body is positioned adjacent the T-box so that the tongue on each end engages a groove of an upper clamp body. The upper strongback body is coupled to each upper clamp body with a cross bolt. The cross-bolt extends through the bolt opening in the flange of the upper clamp body and threadingly engages the bolt opening in the end of the upper strong-back body. Likewise, the lower strong-back body is positioned adjacent the T-box so that the tongue on each end engages a groove of a lower clamp body. The lower strongback body is coupled to each lower clamp body with a cross bolt. The cross-bolt extends through the bolt opening in the flange of the lower clamp body and threadingly engages the bolt opening in the end of the lower strong-back body. The cover plate is then coupled to the upper and the lower strong-back body with cover bolts that extend through bolt openings in the cover plate and threadingly engages the corresponding bolt openings in the upper and the lower strong-back bodies.
The above described T-box clamp apparatus provides structural integrity to the T-box and the joints between the T-box and the distribution header pipes. The T-box clamp apparatus applies compressive forces to the T-box welded joints to hold the joints together in the event that one or more welds fail, and therefore ensures that the core spray system delivers the necessary volumetric flowrate to the reactor core in the event of a loss of coolant accident.